Add new NDVI filter

Basically, the problem is that you are storing the app's char pointer, which is not allowed. It is not safe for the plugin to assume the char pointer is valid beyond the call to f0r_set_param_value(). Not all apps are using something that works like mlt_properties. From frei0r.h:
```
 * If the parameter type is of F0R_PARAM_STRING, then the caller should
 * supply a pointer to f0r_param_string (char**). The plugin must copy
 * copy the string and not assume it exists beyond the lifetime of the call.
 * The reason a double pointer is requested when only a single is really
 * needed is simply for API consistency.
```